Insulating method



April 1961 E. E. THlESSEN 2,979,432

INSULATING METHOD Filed Dec. 15, 1954 3 Sheets-Sheet 1 FOE/W C 0/1 APPLY DQ/ME (0A 7' A ppL Y lNJUL A T/O/V APPL Y PIQfSSMQE cf FZAS/f l UL CAIV/ZE ATTORNEY If j] 41;

April 1951 E. E. THIESSEN 2,979,432

INSULATING METHOD Filed Dec. 15, 1954 3 Sheets-Sheet 2 ,7 I 4 J7 4 W 4!I F I 0/2567 cuppa/r JOUPC'E A NO (ON TQOL INVENTOR AT'TORNEY April 11,1961 E. E. THIESSEN INSULATING METHOD 3 Sheets-Sheet 5 Filed Dec. 13,1954 I N V E N TO R 6222222 6 775252 022 ff/.6040

ATTOQ NEY United States INSULATING METHOD Filed Dec. 13, 1954, Ser. No.474,775

3 Claims. (Cl. 154-s0) This invention relates to insulating systems forelectrical conductors and coils and more particularly to a method of andapparatus for applying such systems. The invention has particularapplication to coils for use as field or armature coils fordynamoelectric machines.

It is well known that the insulation system of electrical apparatus suchas dynamoelectric machines imposes a limit upon the power output ratingof the apparatus because of the deterioration of the insulating materialat high temperatures. This is particularly true of organic insulatingmaterials. Certain silicone compounds, however, have found widespreaduse as electrical insulation and because of their excellentthermalstability are considered as a high temperature insulating material alongwith mica and other inorganic substances. Silicone rubber is admirablyWell suited for insulating systems sub-' jected to severe operatingconditions such as mechanical stresses, abrasion and chemical action. Amost important characteristic of silicone rubber is its ability towithstand indefinitely long exposure to temperatures in the vicinity of180' degrees C. without significant change in its mechanical orelectrical properties. Additionally, silicone rubber is recognized as agood thermal conductor among insulating materials. Thus, silicone rubberinsulation contributes to improved performance of'electrical apparatusby permitting a higher temperature rise and by increasing the thermaldissipation.

However, the prior art insulation systems utilizing silicone rubber,particularly built-up jackets of plural layers and the like, are subjectto the adverse effects of thermal barriers such as voids or air spacesin the insulation system. This arises because of the failure to achievea good bond or adhesion between the silicone rubber and the conductormaterial and between adjacent layers in the system. Consequently, theinsulation system or jacket does not dissipate heat readily.

Therefore, it is an object of this invention to provide a method ofapplying a high temperature insulation system of silicone rubber to anelectrical conductor or coil which will afford improved heatdissipation.

Another object is to provide an insulation system of silicone rubber onan electrical coil which is bonded to the coil conductor andcharacterized by the absence of voids or air spaces and hence improvedthermal conductivity.

Another object is to providea method of insulating electrical coils withvulcanized silicone rubber bonded layer to layer and free of air spaces.

A further object is to provide a method of applying glass clothsupported silicone rubber to electrical coils which provides avulcanized insulation system bonded to the coil conductor and layer tolayer.

An additional object of the invention is to provide apparatus forsimultaneously vulcanizing and pressing silicone rubber insulation on anelectrical coil to produce an insulation of high thermal conductivity.

These and other objects of the invention and the manner in which theyare achieved will become apparent from stem a 2,979,432 Pat ented Apr.11, 1961 ICC the description which follows taken with the accompanyingdrawings in which:

Figure 1 is a plan view of a typical coil to be insulated by theinventive method.

Figure 2 is a diagrammatic representation of the method steps.

Figure 3 is a cross section taken on line 3-3 of Figure 1.

" Figure 4 is a cross section of a coil conductor showing successivestages of application of the insulating material.

Figure 5 is a plan view of vulcanizing and pressing apparatus forperforming a part of the method.

Figure 6 is an elevational view in section of part of the apparatus ofFigure 5.

It has been found that an insulation system of silicone rubber havingexceedingly good properties of heat dissipation may be produced bybonding the silicone rubber to the electrical conductor and by bondingthe insulation layer to layer. According to this invention, the bondingto the electrical conductor is obtained by treating the conductor with aprime coat of varnish and applying the silicone rubber in asemi-vulcanized state. The silicone rubber is then flash-vulcanized byelectrical resistance heating and simultaneously pressed against theconductor. Where plural layers of silicone rubber are used, bonding oflayer to layer is achieved by the simultaneous flash vulcanizing andapplication of pressure.

- The silicone rubber employed in the inventive method is suitably intape or sheet form. It preferably comprises a glass cloth backing ofapproximately 0.007 inch thickness impregnated and coated on one sidewith iron oxide filled, partially vulcanized silicone rubber to a totalthickness of approximately 0.015 to 0.025 inch. The chemistry of thesilicones including silicone rubber is treated at some length in theliterature and will not be discussed herein. See for example, Rochow,Chemistry of the Silicones, 2nd ed.,-Wiley and Sons, Inc, 1951, and theliterature cited therein. Very briefly, silicone rubber is a syntheticmaterial, the molecules of which are long chains of silicon-oxygen'units with organic groups attached to each silicon atom.

A specific silicone rubber tape and sheet which is preferred for theinventive method is a polymeric organosiloxane on glass cloth backingsold under the trademark Silastic R by the Dow-Corning Corporation. Thephysical properties of this material are very well suited for coilinsulating jackets which must Withstand high operating temperatures,mechanical stresses, abrasion and exposure to chemicals. Typicalproperties are as follows:

Tensile strength unvulcanized, pounds per inch Width (15 mil thickness)Water absorption (24 hrs. immersion at 25 degrees C.) 0.5 to 1.0

Thermal stability:

Dielectric strength, volts per mil inch electrode)- c No aging 863 Aged7 days at 523 degrees F. 793 Compression set percent 4 to 6 humidity) 21o To insure superior bonding of the silicone rubber in the process tobedescribed subsequently, the silicone rubber should meet certaininitial conditions. The rubber should be partially vulcanized or curedonly sufficiently to prevent adhesion between layers on the rolls oradjacent sheets. The rubber surface and glass backing must be free fromdirt and other foreign material.

The inventive method as practiced in a typical application for theinsulation of a motor coil will be described in detail with reference tothe drawings. In Figure 1 there is shown an interpole coil 1 for atraction motor which is to be provided with an insulating system orjacket in accordance with the present invention. The coil 1 is formedfrom a coil conductor of rectangular, metallic copper strap 3 and isprovided with leads 4 and 5. The method steps performed in this specificapplication are enumerated in the diagram of Figure 2.

Referring now to Figure 3, after the coil has been wound from the copperstrap 3 and the leads have been formed, the individual coil conductorturns 6 are insulated from each other by inserting any suitableinsulating material preferably of the high temperature class such asmica plates 7. The mica plates 7 are of a'width such that they are flushwith the inside walls 8 and outside Walls 9 of the coil 1.

After the coil is formed and insulated turn to turn as described above,all exposed surfaces including inside surface 8, outside surface 9,upper axial surface 10 and lower axial surface 11 of the coil conductorare given a prime coat of shellac varnish 12. The coil is then bakeduntil the resoftening temperature of the varnish 12 exceeds thetemperatures of subsequent processing or operation which will appearhereinafter. In the specific example a suitable baking time is 5 hoursuntil the varnish is cured such that it is not thermoplastic at 190degrees C. The varnish found to be most suitable for obtaining a goodbond of the subsequently applied layers of silicone rubber is preparedas a solution or cut of dry lac resin and denatured alcohol or othersuitable solvent. Preferably, the shellac varnish is that known in theart as grade A, orange, of light body and approximately a four poundcut. The shellac portion of the solution should constitute approximately34.5 percent of the solution and contain substantially no rosin, amaximum of 1.25 percent of insoluble matter, 5.5 percent of wax, and 1percent of ash.

Following the baking of the varnish on the coil conductor surfaces,silicone rubber tape 13 is applied as shown in Figure 4 which representsa plan view in section of a coil turn 6. The tape is applied with thesilicone rubber coating adjacent the varnish coat and is suitablywrapped in half-lapped fashion providing a double layer for eachwrapping. Multiple layers may be provided by an'additional wrapping ofsilicone rubber tape 13 if desired. A tight wrapping of the tape is verydesirable and may be insured by compressing the coil turns 6 as thetaping progresses. To provide an additional layer of silicone rubberinsulation on the inside wall 8 of the coil 1 a silicone rubber sheet 14is wrapped on the inside wall 8 and folded over the outside wall 9. Anadhesive tape 15 of any suitable kind such as glass cloth tape is thenwrapped tightly about the coil 1 to complete the insulating jacket 16.

Following the application of the insulating material to the coil, theentire surface of the insulating jacket 16 is pressed tightly againstthe coil conductors 6 and is subjected to what is termed herein as flashvulcanizing. The flash vulcanizing is effected by the application ofheat in such manner that a very high rate of temperature rise isproduced. The temperature for vulcanizing, suitably in excess of 120degrees C. and preferably about 175 degrees (3., must be reached in ashort period of time, preferably less than 7 minutes to achieve thedesired result of an intimately bonded, void free, flexible and toughinsulating jacket. If the rate of heating is less than a certaincritical minimum the rubber becomes a gummy or viscous, fluid-like mass.Prior to or simultaneously with the application of vulcanizing heat,pressure must be applied to the insulating material. Although the valueof the pressure is not critical, it has been found that approximately 50pounds per square inch is quite satisfactory. The pressure is effectiveto insure good heat transfer during vulcanization and elimination of allair spaces in the completed insulating jacket.

At this point in the process, the silicone rubber tape 13 and 13 andsheet 14 are bonded intimately together and the inner layer of tape 13-is bonded intimately to the coil conductor 6. It has been found that theinterposition of the organic varnish coat 12 between the coil conductor6 and the silicone rubber is of great importance to superior bonding.Even though the organic material, in a high temperature insulationsystem, eventually deteriorates from thermal aging, no detrimentaleffect on the bond results. An exact explanation of this behavior andthe manner in which the organic varnish contributes to the bonding ispresently unknown. It has been advanced as a theory that shellac varnishduring the baking period becomes somewhat porous and forms an acid, bothof which conditions contribute to the bond.

Following the flash vulcanizing and simultaneous pressing of theinsulation jacket 16 the insulated coil is subjected to an after-cure bybaking to develop the optimum electrical and mechanical properties ofthe insulation. This baking may be suitable carried out in an aircirculating oven at a temperature higher than subsequent operatingtemperatures of the coil. In the specific application described hereinbaking at degrees C. for a period of 16 hours is most satisfactoryalthough it will be understood that the baking time will vary with thesize and shape of the coil and the insulation thickness. Subsequentcoatings of protective resins or the like may be applied if desired.

Simultaneous application of pressure to all surfaces of the insulatingjacket and flash vulcanizing of the silicone rubber may be most suitablyaccomplished in the vulcanizing and pressing apparatus of Figures 5 and6. The vulcanizing and pressing apparatus comprises a table 20 adaptedto be raised and lowered on shafts 21 by a hydraulic actuator 22 (Figure6). A lower press 23 is mounted on a slidable tray 24 provided withoperating handle 25. The lower press 23 has a resilient facing 26,suitably of rubber, which is adapted to support the coil 1 thereon inengagement with the insulating jacket 16 at the axial surface 11 of thecoil. In axial alignment with lower press 23 is upper press 27 dependingfrom plate 28. Upper press 27 is provided with a resilient facing 29adapted to engage the insulating jacket 16 at the axial surface 10 ofcoil 1 when the press 23 is moved with table 20 from the lowered or openposition to the raised or closed position.

A compressible block of resilient material 30, suitably of rubber, ismounted on lower press 23 by any suitable securing means such as bolt 31extending through washer 32 and sleeve 33. A resilient plug 30' isfitted over bolt 32 to provide a continuous surface on block 30. Asshown in Figure 5 the rubber block 30 has a cross sectionalconfiguration conforming to the inside surface 8 of the coil 1. Therubber block is adapted to extend axially through and protrude beyondthe coil 1 and with the presses 23 and 27 in the open position the blockterminates at 34. With the presses 23 and 27 in the closed position therubber block 30 is compressed therebetween and terminates at 34'. Axialcompression of block 39 causes it to expand laterally and to exertpressure against the insulating jacket 26 at the inside surface 8 ofcoil 1.

Mounted on table 20 are side presses. 35 and 35' for compressing theinsulating jacket 16 against the outside lateral surface 9 of thecoil 1. For purposes of illustration only side press 35 is shown in theopen position and side press 35 is shown in the closed position. Sincethe'two side presses are identical, the description will be given withreference to press 35 although the same reference numerals with primesymbols affixed will apply to corresponding parts of press 35'. Acontinuous resilient facing 36 is mounted on pressing shoes 37, 38 and39 which have a configuration conforming to that of the adjacent outsidesurface 9 of the coil. The pressing shoes 37, 38 and 39 are connected byplungers 40, 41, and 42 to hydraulic actuators 43, 44 and 45,respectively. The shoes 37 and 39 are preferably pivotally connected tothe respective plungers 40 and 42 to permit the necessary angulardisplacement between the open and closed position of the press 35.Mounted on the tray 24 are end presses 46 and 46'. End press 46comprises resilient facing 47 mounted on pressing shoe 48 which is con-,nected by plunger 49 to hydraulic actuator 50 for movement from open toclosed positions. Similarly end press 46' includes resilient facing 47,mounted on pressing shoe 48 which is connected by plunger 49' tohydraulic actuator 50.

The hydraulic actuators are of any suitable type, although preferablyprovided with double acting pistons energized through conduits 51 and 52to provide controlled closing and opening of the presses. The conduitsare connected to a hydraulic source and control system (not shown) whichwill not be described since it forms no part of the present inventionand suitable arrangements are well known to those skilled in the art.

The coil leads 4 and extend outwardly from the coil 1 to a positionadjacent end presses 46 and are connected through conductors 53 and 54to an electrical source 55 provided with suitable control arrangementsto regulate the current through the coil 1. The electrical source ispreferably of direct current and since the detailed structure thereofforms no part of this invention, it is represented in block diagram inthe interest of clarity.

The electrical source must be capable of delivering a suflicient currentto insure the proper rate of temperature rise in the coil to produceflash vulcanization of the silicone rubber. The current required is, ofcourse, dependent upon numerous factors including the size, shape,insulation thickness and conductor material of the coil being processed.For coils of large mass such as traction motor coils, electricalresistance heating is exceptionally well suited to attain the requiredheating rate and such coils necessarily require a high value of heatingcurrent. As a typical example, an interpole coil for traction motors of14 turns of copper strap having a cross section of approximately 0.75inch by 0.34 inch and approximate total length of 500 inches in aninsulating jacket substantially that shown in Figure 4 requiresapproximately 1600 amperes to raise the coil temperature to 175 degreesC. in 7 minutes or less.

The operation of the vulcanizing apparatus of Figures 5 and 6 will nowbe apparent. The coil 1 provided with the insulating jacket 16 ofsilicone rubber is positioned on lower press 23 coaxially of rubberblock 36. Leads 4 and 5 are connected to source 55 by conductors 53 and54. Pressure is admitted to the hydraulic actuators and the lower press23 is raised by hydraulic actuator 22 to press the upper and lowersurfaces of the coil insulating jacket 16 between resilient facings 29and 26. The rubber block 30 is compressed axially between presses 23 and27 and is expanded laterally thereby exerting pressure on the insidesurface of the insulating jacket. At the same time, the presses 35, 35',46 and 46 are closed by the associated hydraulic actuators and exertpressure on the outside lateral surface of the coil insulating jacket.Substantially. simultaneously with the closing of the presses, the coilis electrically energized from source 55 with the requisite heatingcurrent. After the heating current and pressure have been applied for apredetermined period of time, the current is interrupted and the pressesare opened. The coil 1 is removed for the subsequent processingoperation of baking and the vulcanizing apparatus is ready for a newcycle.

The advantages achieved from the inventive process and apparatus willnow be apparent. The process of application is comparatively simple andpermits the use of commercially available insulating materials. Theinsulated coil is capable of withstanding operation at high temperatureand readily dissipates internally generated heat. The high rate of heatdissipation is attributed to the absence of voids or air spaces in theinsulating jacket and the intimate bonds between the silicone rubberlayers and coil conductor. The heat dissipation is further enhanced bythe relatively high thermal conductivity of the silicon rubber itself.The finished insulating jacket exhibits excellent properties ofmechanical toughness and resilience and thus is capable of withstandingsevere mechanical stress imposed by vibration, thermal expansion andhigh currents in the coil. Additionally the insulating jacket isresistant to abrasion, water absorption and chemical action.

This invention has been described with respect to a particularembodiment for illustrative purposes only and such embodiment is not tobe construed in a limiting sense. Numerous modifications and variationswithin the spirit and scope of the invention will now occur to thoseskilled in the art. For a definition of the invention, reference is madeto the appended claims.

I claim:

1. The method of insulating an electrical conductor of metallic copperfor operation ata high temperature comprising coating said conductorwith a shellac varnish, curing said varnish on said conductor until itsresoftening temperature exceeds said high temperature, applying a layerof partially vulcanized, glass cloth supported silicone rubber over saidvarnish, applying pressure to the surface of said rubber, andvulcanizing said rubber by passing a current through said conductor.

2. The method of making a void-free insulating jacket for an electricalcoil having plural conductor turns of metallic copper to be operated ata high temperature comprising inserting an insulation material betweenadjacent turns, coating the exposed surfaces of said conductor turnswith shellac varnish, curing said varnish until its resofteningtemperature exceeds said high temperature, applying a layer of glasssupported partially vulcanized silicone rubber over said varnish,compressing said rubber against said coil, passing a current throughsaid coil to vulcanize said rubber, and baking said coil in an aircirculating oven at a temperature exceeding said high temperature todevelop optimum properties of said insulating jacket.

3. The method of making a void-free insulating jacket for an electricalcoil having plural conductor turns of metallic copper to be operated ata high temperature comprising inserting insulating material betweenadjacent turns, coating the exposed surfaces of said conductor turnswith orange shellac varnish, curing said varnish until its resofteningtemperature exceeds said high temperature, applying plural layers ofglass supported partially vulcanized silicone rubber over said varnish,compressing said rubber against said coil, passing a current throughsaid coil to effect vulcanization of said rubber at a temperature ofdegrees C. within seven minutes, and baking said coil in an aircirculating oven at a temperature exceeding said high temperature todevelop optimum properties of said jacket.

References Cited in the file of this patent UNITED STATES PATENTS1,874,723 Dawson Aug. 30, 1932 2,454,625 Bondon Nov. 23, 1948 2,473,842Askey June 21, 1949 (Othefreferences on following page) UNITED STATESPATENTS DOrio Apr. 25, 1950 Coggeshall Apr. 24, 1951 Botts et al. June24, 1952 Smith-Johannsen June 24, 1952 Smith-Johannsen June 24, 1952Ford Dec. 30, 1952 Dexter Apr. 13, 1954 8 Bram Nov. 2, 1954' Blaisdellet a1. Jan. 24, 1955 Collings May 17, 1955 Keil Ian. 24, 1956 OTHERREFERENCES October 1951, pages 134137.

UNITED STATES PATENT OFFICE CERTIFICATION OF CORRECTION Patent No, 2 979432 April 11 I961 Elmer E, Thiessen It is hereby certified that errorappears in the above numbered patent requiring correction and that thesaid Letters Patent should read as corrected below.

Column 2 line 54 for 005 to 1,0 read 005 to 1.0% column 4 line 27 for'suitable" read suitably column 6, line 14 for "silicon" read siliconeSigned and sealed this 5th day of September 1961 Att ERNEST W. SWIDERDAVID L. LADD Attesting Officer Commissioner of Patents UNITED STATESPATENT OFFICE CERTlFICATlN 0F COEUHN Patent No, ,2 979 432 April ll 1961Elmer Eo Thiessen It is hereby certified that error appears in the abovenumbered patent requiring correction and that the said Letters Patentshould read as corrected below.

Column 2 line 54L for "005 to 1.0" read 05 to 1.0% column 4 line 27 forsuitable read suitably column 6 line 14 for 'silicon" read silicone e.

Signed and sealed this 5th day of September 19610 Att zs t ERNEST W.SWIDER v DAVID L. LADD Attesting Officer Commissioner of Patents

3. THE METHOD OF MAKING A VOID-FREE INSULATING JACKET FOR AN ELECTRICAL COIL HAVING PLURAL CONDUCTOR TURNS OF METALLIC COPPER TO BE OPERATED AT A HIGH TEMPERATURE COMPRISING INSERTING INSULATING MATERIAL BETWEEN ADJACENT TURNS, COATING THE EXPOSED SURFACES OF SAID CONDUCTOR TURNS WITH ORANGE SHELLAC VARNISH, CURING SAID VARNISH UNTIL ITS RESOFTENING TEMPERATURE EXCEEDS SAID HIGH TEMPERATURE, APPLYING PLURAL LAYERS OF GLASS SUPPORTED PARTIALLY VULCANIZED SILICONE RUBBER OVER SAID VARNISH, COMPRESSING SAID RUBBER AGAINST SAID COIL, PASSING A CURRENT THROUGH SAID COIL TO EFFECT VULCANIZATION OF SAID RUBBER AT A TEMPERATURE OF 175 DEGREES C. WITHIN SEVEN MINUTES, AND BAKING SAID COIL IN AN AIR CIRCULATING OVEN AT A TEMPERATURE EXCEEDING SAID HIGH TEMPERATURE TO DEVELOP OPTIMUM PROPERTIES OF SAID JACKET. 